The present disclosure relates to color changing inkjet ink compositions and methods of printing on hygienic articles.
Methods have been proposed to indicate wetness in a hygienic article, namely diapers or incontinence pads. For example, the inner surface (or the surface towards the body) of the outer liner of a diaper may have a printed message such as animal figures, alphanumerics, or other patterns attractive to children (infants or toddlers). When the diaper becomes wet and the urine reaches the outer liner, the figure or pattern in the printed message changes. Such changes in the image will be visible to the wearer or caregiver, thereby indicating that a diaper change is required.
To function as an effective wetness indicator, the printed image needs to be suitably attractive to the wearer, provide a first image state and provide a visually distinguishable second image state after incontinence, wherein one or both states should exhibit a high level of contrast. It is often desirable that the second state is darker and even more visible than the first state, so that it is obvious that an action needs to be taken. Since in this case the second state is a positive indication of incontinence, it is preferred that the printed image does not dissolve away, does not revert to the initial state, and remains sharply defined and discernible. The first state also needs to be very stable and not change prior to the incontinence event, even when exposed to, for example, very humid environments.
Currently, wetness indicators that change from lighter to darker image states are provided by hot melt applicators printing glue-like compositions. Due to the relatively large inner bore diameter of the nozzles required to deliver these thick glues, the applied images offer only crude resolutions, no graphics or text are effectively possible, and the indicator glue usages are higher than desired. Another technology used to apply wetness indicators is flexographic contact printing. Flexographic printers, however, cannot deliver images with digital variable information (the ability to change images from one print to the next), require a relatively large footprint on a production line, and thus are not preferred for reasons of inconvenience and cost of integration. For these reasons, flexographically printed hygienic articles are often preprinted in large production batches in secondary processes, which adds cost and complexity to manufacturing.
In contrast, ink jet printing is a well-known technique by which digital printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Inkjet offers the general advantages of high resolution, digital variable information, the ability to deliver relatively controlled doses of fluid, low consumable use, low VOC emissions, and ease of integration. Ink jet printing can be broadly divided into drop-on-demand (DOD) printing and continuous inkjet (CIJ) printing. Binary array, a subset of CIJ inkjet printing, is particularly preferred because it can print at very high production linespeeds in comparison with DOD printing technologies (particularly on a per-printhead basis) and employ solvent based inks exhibiting fast drying rates that enable very fast production speeds on commonly employed web-based production lines while also providing the aforementioned ease of integration.
In general, an ink jet ink composition must meet certain strict requirements to be compatible with ink jet printing systems. These requirements relate to the physical properties of the fluid (such as viscosity), the chemical properties (such as the solubility and compatibility of the components), and the ability to suitably wet the substrate. Further, the ink must be quick drying and smear resistant, and be capable of passing through the ink jet nozzle(s) without clogging, and permit rapid cleanup of the machine components with minimum effort. The selection of fast drying, durable polymers for inkjet inks requires both strong theoretical understanding of these properties as well as empirical validation of their performance. Binary array printing furthermore requires the use of polymers with a highly specific set of molecular properties. Also, binary array like CIJ generally employs an electrostatic mechanism to control drop placement and thus requires an ink composition that is sufficiently conductive and chargeable. At present, there are no color changing inkjet compositions that provide the performance as described above as well as compatibility with desired printers. In fact, it is highly difficult to formulate an inkjet ink with low viscosity, low solids and high volatility that also provides images with good ambient color stability and rapid color change.
Current ink jet formulations that function as wetness indicators rely on a mechanism that washes the ink away from the surface of the printed article. These inks include water-soluble ingredients and as they wash away after contact with bodily fluids the images will change from a darker to a lighter perceived color. In such current wetness indicators the final image is not usually completely removed even after a long period of aqueous exposure, resulting in a muddled and unattractive appearance as opposed to being easy to interpret by the wearer or caregiver.
Hence, there is generally a need for inkjet formulations and methods that enable the online application of effective color changing wetness indicators with more attractive patterns, more positive and safe indicating end-points, reduced materials use, and that are able to be easily integrated, as compared with the prior art methods.